Support multiple returns in a function

[oxisto]

We are starting to support languages that support multiple returns and assignments of return values, such as Golang and Python.

How should we deal with them? Currently, for our experimental Golang support, we only take the type of the first return value. Should we introduce a TupleType which then holds the individual types? We also then need to take this into consideration when the individuals of a tuple are assigned to a variable like a, b := myFunc().

Any ideas, @konradweiss, @maximiliankaul?